dictionary REFACTOR unify functions pattern to return LY_ERR

Change API functions manipulating with libyang dictionary to return
LY_ERR value as a primary indicator of the result and the value from the
dictionary is returned via optional output parameter.
diff --git a/src/plugins_exts_nacm.c b/src/plugins_exts_nacm.c
index 7e353ff..5be0619 100644
--- a/src/plugins_exts_nacm.c
+++ b/src/plugins_exts_nacm.c
@@ -103,7 +103,7 @@
                 inherited->parent = iter;
                 inherited->parent_type = LYEXT_PAR_NODE;
                 if (c_ext->argument) {
-                    inherited->argument = lydict_insert(cctx->ctx, c_ext->argument, strlen(c_ext->argument));
+                    LY_CHECK_RET(lydict_insert(cctx->ctx, c_ext->argument, strlen(c_ext->argument), &inherited->argument));
                 }
                 /* TODO duplicate extension instances */
                 inherited->data = c_ext->data;